The present invention relates to an electromagnetically actuated fuel injection device for an internal combustion engine.
A device of this type substantially comprises a body of elongate form, an injection nozzle fixed to the body and provided with at least one fuel injection orifice and a shutter needle movable axially within the interior of the body and of the nozzle between a closure position in which it closes the said orifice and an open position in which it leaves the passage through the orifice itself unobstructed.
This device further includes a valve body housed within the interior of the said body of elongate form and in which there is formed a control chamber into which fuel under high pressure is supplied through a fuel inlet aperture and from which this fuel is discharged through a discharge aperture; the said valve body is disposed above the said shutter needle which delimits the lower part of the first-defined control chamber.
An electromagnetically actuated pilot valve also forms part of a device of this type, this valve being operable to control the passage of fuel through the discharge aperture; such a valve substantially comprises a shutter member operable to close the discharge aperture, the shutter member being fixed to an armature in the form of a disc, urged towards the discharge aperture by the action of at least one spring and adapted to be attracted by a core of an electromagnet fixed to the body of elongate form to cause the shutter member to move away from the discharge aperture.
The present invention relates to an improvement of the above-mentioned pilot valve which forms part of an electromagnetically actuated fuel injection device.
Pilot valves of the type briefly described have several disadvantages.
First of all some surfaces of the first mentioned valve body in which the control chamber and the fuel inlet and discharge apertures are formed can not be worked with a high surface finish and with close tolerances; this is true in particular of the surface onto which the said discharge outlet opens and against which the shutter member of the pilot valve rests; consequently the function of this valve can be defective, or else to obtain a working finish and acceptable tolerances complex and expensive working operations are required.
Moreover the members which are utilised to fix the valve body to the body of elongate form for the purpose of resisting the high force which must be exerted on the second body and which is generated by the high fuel pressures, is not very reliable. To this end an annular threaded ring nut is usually used, which is fixed to the head of the injector and which has an internally threaded section which is screwed on to a corresponding externally threaded section of the body of elongate form: damage in fact occurs in the connection region of this ring nut with the head since the material of the side wall of the head itself has a low mechanical strength because of its magnetic permeability.
Moreover, when the armature is attracted by the core, a surface of the armature itself comes into contact with a corresponding surface of the core; consequently, because of the residual magnetisation of the armature and the core, the first tends to remain in contact with the second even when the excitation current of the electromagnet ceases: the separation between these two members, which takes place by the action of the first mentioned spring, occurs with a certain delay thereby detrimentally affecting the speed of response of the device.
Again, because of the direct contact between the surface of the armature and that of the core, a certain wear of these surfaces takes place, in particular that of the core, with the disadvantage of varying from the initial calibration conditions of the device; moreover, since this wear is not normally uniform, when the armature comes into contact with the core, it assumes an incorrect position, thereby forming a small angle between the axis of the armature and the axis of the core, with the consequence of detrimentally affecting the seal formed by the pilot valve in its closure position.
Finally, since the stroke of the pilot valve shutter member is determined uniquely in the end of stroke position of the armature with respect to the core it is not possible to effect an adjustment of the opening movement of this valve and therefore to define, in a rigorous way, the working conditions of the device on which the valve is mounted.